Thin film coated, intensity based fiber-optic probes for the detection of fluorescence have found many applications due to their small size and versatility. These applications include chemical research, biomedical research and clinical surgery. Other applications include waste water monitoring and explosive detection as well as the detection of leaks from containers of corrosive liquids and the like.
Fluorescent signals are usually very weak and disperse in all directions. Measurement of these signals relies heavily on the light collection efficiency of the measurement device.
A first known type of probe comprises a single optical fiber having a core covered with a film. When illuminated, the film emits light via fluorescence. Typically, the core is only covered at an end portion of the fiber (i.e. the probe tip) and light collected by the probe is measured to determine the presence of certain materials or effects. These measurements are based upon collected fluorescent light as a result of evanescent waves existing in the fiber cladding area (and beyond). The amount of collected fluorescent light will change if the refractive index of the film changes as a result of being in contact with those materials or effects (for example if the film absorbs liquid).
A second known type of probe comprises an optical fiber (or multiple optical fibers including at least one illuminating fiber and at least one receiving fiber) having a film coating attached to the end face of the receiving fiber or placed at a certain distance from its end face. Again, light collected by the probe is measured to determine the presence of certain materials or effects.
Careful consideration of the refractive index of film coatings for known probe designs is required to meet the guiding condition of the fiber. Often, the cladding of the receiving optical fiber has to be removed and replaced with certain specific materials possessing a refractive index lower than that of the fiber core material to match the guiding condition of the receiving optical fiber.